Semiconductor qubits remain a leading candidate technology for quantum information processing. In such qubits, spins can have extremely long quantum coherence due to a decoupling of spin information from charge noise in many materials. Moreover, semiconductor qubits can be relatively small, thereby enabling high density. However, semiconductor qubits typically rely on microwave pulse control in performing one or more gate operations, which can result in slow gates with significant potential for crosstalk with nearby qubits.
In some semiconductor qubit systems, the exchange interaction has been used to provide a natural and fast method for entangling semiconductor qubits. The exchange interaction can be used to perform two-spin entangling operations with a finite-length voltage pulse or to couple spins with a constant interaction. Exchange also provides a solution to the control problem by allowing a two-level system to be encoded into the greater Hilbert space of multiple physical spins.
Decoherence free subspaces and subsystems (DFS) were developed based on this exchange interaction, and many multi-spin-based qubits have been proposed and demonstrated with various desirable properties for quantum computing based on the exchange interaction concept. Such examples include 2-DFS (also known as “singlet-triplet” qubit), 3-DFS (also known as “exchange-only” qubit), and 4-DFS qubits. Such DFS configurations can allow for gate operations via a sequence of pair-wise exchange interactions between spins with fast, baseband voltage control of metallic top-gates. While these DFS configurations are immune to global field fluctuations to some extent, their fidelity is limited by charge noise, since charge and spin are coupled when spins undergo exchange.
Another proposed multi-spin qubit, known as a “resonant exchange” (RX) qubit, employs an encoded qubit made of 3 quantum dots (QD) with “always-on” exchange interactions. For the RX qubit, the triple QD device is tuned to be in a regime where the (111) configuration is close to (201) and (102) configurations for initialization and readout. This implies that εM is relatively large, comparable to the on-site Coulomb interaction U (see RX regime shaded in gray in FIG. 2D). In other words, the middle QD has a much higher chemical potential (i.e., local potential) than the respective chemical potentials for the outer QDs. As a result, the RX qubit is only partially insensitive to noise.
For example, the first derivative of the RX qubit frequency vanishes for one of the two detuning parameters that are affected by charge noise. The qubit is maintained at this parameter space while microwave control allows for single qubit operations “resonant” with the gap of the 3-spin system. For two qubit gates, the RX qubit offers a relatively large transition dipole matrix element for two-qubit dipole-dipole coupling, either directly or through a resonator. Unfortunately, complete insensitivity for an RX qubit with respect to both detuning parameters (e.g., ε and εM) only lies outside of the (111) singly occupied regime, where higher order effects otherwise limit the coherence of the qubit.
Furthermore, the partial insensitivity of the RX qubit depends on the asymmetry of the tunnel coupling between the QDs of the qubit, requiring a different method (i.e., microwave control) to implement full single qubit rotations. Thus, the logical single qubit gate operations are implemented using microwave at a partially insensitive parameter location defined by the tunnel coupling between QDs.
Embodiments of the disclosed subject matter may address one or more of the above-noted problems and disadvantages, among other things.